Coaxial drive systems are well known and are commonly employed where it is desired to reduce undesired gyroscopic or rotational moment forces acting on the driven vehicle or device. For example, underwater vessels such as torpedoes and submarines often employ counter-rotating coaxially arranged propellers so that the tendency for the vessel to rotate with respect to a fixed propeller, rather than the propeller rotating with respect to the fixed vessel, is reduced. Similarly, some airplanes employ counter-rotating propellor systems to reduce the effects of propellor torque.
However, in all known coaxial drive systems, one drive is achieved via a first drive shaft which is hollow (which connects a prime mover to the first driven member, such as a propeller) and the other drive is achieved by a second drive shaft, located within the hollow first drive shaft, which connects a prime mover to the second driven member.
While such systems have been employed for many years, they suffer from disadvantages in that the required hollow drive shaft tends to be large and expensive to fabricate, the weight and/or mass of the drive shafts tends to be large and the configuration and arrangement of the overall coaxial system tends to be constrained by the requirements for the nested drive shafts.
Advanced helicopter designs have employed counter-rotating, coaxial rotor systems to reduce or eliminate the need for a tail rotor and/or to reduce helicopter operating noise. In flight-related coaxial designs, such as those employed for helicopters, the weight of the nested drive shafts and the mechanical complexities of mounting such systems and operably connecting them to respective prime mover systems has resulted in expensive and/or complex systems which can be fragile and/or difficult to service and/or maintain.
More recently, small helicopter systems, such as radio controlled or autonomous drones have employed coaxial drive systems wherein the prime movers are electric motors. However, such systems still suffer from the disadvantages of requiring coaxially arranged drive shafts to which the helicopter rotors can be attached. Such systems also require appropriate transmissions and/or complex linkages to transfer the torque produced by the electric motors to the drive shafts and then to the rotors and such transmissions and linkages increase cost, weight and complexity, which can reduce the reliability of such systems.